1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Conventionally, there has been known an optical scanning device, in which light beam is deflected by an optical deflecting unit, and the deflected light beam is focused on a scanning surface as a fine spot beam, so that the scanning surface is scanned at a constant speed in a main scanning direction, and is applied to a latent-image writing unit in an image forming apparatus such as a laser printer, a laser-beam plotter, a facsimile machine, and a digital copying machine. The optical scanning device writes an image on the scanning surface by deflecting a laser beam emitted from a laser light source, such as a semiconductor laser, by the optical deflecting unit to scan the scanning surface such as an image carrier, and at the same time, with intensity modulation of the laser light source based on image signals.
As the optical deflecting unit, a polygon mirror that rotates at a constant velocity is widely used. As a scanning lens that focuses the deflected light beam on the scanning surface, an f0 lens is generally used. The f0 lens makes the scanning speed of the deflected light beam at each scanning position on the scanning surface substantially constant, and the beam spot size substantially uniform.
Meanwhile, a micromirror that uses micromachining technology to generate sine wave oscillation of a resonant structure has been proposed. The polygon mirror is a large device, and accompanies mechanical high-speed rotation. Therefore, there are problems of banding due to vibration, temperature rise, noise, and power consumption increase. On the other hand, if the micromirror is used as the deflecting unit in the optical scanning device, the device is downsized, and the above problems can be considerably reduced.
By using the micromirror that generates sine wave oscillation instead of the polygon mirror, low noise and low power consumption can be realized, thereby enabling to provide an image forming apparatus suitable for an office environment.
Furthermore, by using the micromirror that generates sine wave oscillation instead of the polygon mirror, an environmentally-friendly image forming apparatus can be provided due to its lower power consumption.
However, the conventional technique has the following problems. That is, in the optical scanning device using the polygon mirror and the f0 lens, the scanning speed at each scanning position on the scanning surface may not be completely corrected until constant speed scanning is realized over a whole active writing area. The lack of correction can occur at the time of reducing the thickness of the scanning lens for cost reduction. Generally, in the optical scanning device using the polygon mirror and the f0 lens, a static beam spot size (diameter of light beam in an arbitrary image height on the scanning surface) at each scanning position is substantially constant. In this case, if a light source is modulated with the same emitted pulse width at each scanning position, as the respective scanning speeds become different, scanning distances become different, and as a result, the scanning beam spot sizes become different. The difference in spot size of the scanning beam that exposes the surface of a photoconductor at respective scanning positions generates a difference in a dot diameter and density in each area of the image, thereby leading to degradation of the image.
On the other hand, when the micromirror that generates sine wave oscillation is used as the deflecting unit, a deflecting angle sinusoidally changes. Therefore, when the f0 lens used for the current writing optical system is used for a scanning imaging optical system, the scanning speed becomes slow in a peripheral image height, and therefore the scanning speed on the scanning surface is not constant. Also in this case, degradation of the image occurs.
With respect to this problem, Japanese Patent Application Laid-open No. 2005-215571 describes an optical scanning device having a wide effective writing width, and that an excellent scanning constant-speed characteristic can be obtained by using a scanning imaging optical system (f·arcsin lens) having an imaging characteristic (f·arcsin characteristic) as shown by the following equation:H=K×sin−1(φ/2φ0)where H denotes an image height, K denotes a constant of proportion, φ denotes a deflection angle, and φ0 denotes an amplitude, and by optically correcting a waist position of main scanning light beam. However, if the above optical correction is performed, a deviation of the spot size of the main scanning light beam between image heights increases on the scanning surface, thereby causing degradation of image quality.
In the optical scanning device using the micromirror that generates sine wave oscillation as the deflecting unit, there is a tradeoff relationship between the scanning constant-speed characteristic and the deviation of the spot size of the main scanning light beam on the scanning surface between image heights, and an optical scanning device having the excellent scanning constant-speed characteristic and deviation of the spot size between image heights to form an excellent image cannot be provided.
Japanese Patent Application Laid-open No. 2002-258204 and Japanese Patent Application Laid-open No. 2002-82303 describe an optical scanning device using a micromirror that generates sine wave oscillation as a deflecting unit, where optical correction of the waist position of a main scanning light beam is not performed by using the optical scanning device having an imaging characteristic as shown in the above equation, but a deflection angle of the micromirror is decreased with respect to the maximum amplitude. In the deflection speed in this case, a linear change is predominant rather than a sinusoidal change, to obtain both an excellent scanning constant-speed characteristic and an excellent deviation of the spot size of the main scanning light beam between image heights. However, because the deflection angle of the micromirror is decreased, the size of the optical scanning device increases to obtain an effective writing width required at the time of forming the image, thereby restricting the size of the apparatus.